III–V FET channel designs for high current densities and thin inversion layers

Abstract

III–V FETs are being developed for potential application in 0.3–3 THz systems and VLSI. To increase bandwidth, we must increase the drive current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> = qn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> v <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">inj</inf> W <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> per unit gate width W <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> , requiring both high sheet carrier concentrations n <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> and high injection velocities v <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">inj</inf> . Present III–V NFETs restrict control region transport to the single isotropic Γ band minimum. As the gate dielectric is thinned, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> becomes limited by the effective mass m*, and is only increased by using materials with increased m* and hence increased transit times. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> The deep wavefunction also makes Γ -valley transport in low-m*materials unsuitable for < 22-nm gate length (L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> ) FETs. Yet, the L-valleys in many III–V materials <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have very low transverse m <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</inf> and very high longitudinal mass m <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> . L-valley bound state energies depend upon orientation, and the directions of confinement, growth, and transport can be chosen to selectively populate valleys having low mass in the transport direction <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3,4</sup> . The high perpendicular mass permits placement of multiple quantum wells spaced by a few nm, or population of multiple states of a thicker well spaced by ∼10–100 meV. Using combinations of Γ and L valleys, n <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> can be increased, m* kept low, and vertical confinement improved, key requirements for <20-nm L <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> III–V FETs.